With the invent of the composite propeller blade, which are lighter than the earlier metal and/or metal-composite hybrids, blades in operation experience reduced centrifugal loading. Accordingly, the loading is typically insufficient to prevent rocking caused by high bending loads. Many current designs use a retention assembly that includes one or more bearings that must be preloaded by some means. The rocking effect caused by the bending loads is alleviated in many current designs by use of pitch change bearings with increased diameters. Often, these designs also use extra rows of bearings. However, these additions add to the weight and complexity of the blade and retention assembly design, thereby increasing costs and potential for mechanical failure. In addition, many composite propeller blades attach through a shear bond joint to a metal retention member. This is an inefficient means for load transfer and requires a secondary mechanical backup.
As indicated, the existing solution to prevent rocking is to use a preloaded bearing assembly so as to provide a compensating load. For example, the double row bearing design, used by the assignee of the present application, is preloaded by a large mechanical nut through a very stiff load path, making the application of preloading somewhat difficult. In addition, it is difficult to monitor and retain the initial preload over long periods of service time. Also, current designs require the use of special tools to apply the high preload necessary. And, some form of load monitoring is required to prevent the loss of foundation/attachment stiffness at the base of the blade. Without such monitoring and means for adjusting the preload, a potentially dangerous change in blade resonant frequency can arise undetected. And, since the most current designs use a mechanical nut, small amounts of fretting or wear on the threads that engage the nut can lead to this loss of stiffness. In addition to the above, replacement of blades with these types of retention designs is cumbersome and requires a significant amount of time. U.S. Pat. Nos. 4,850,801 to Valentine and 5,118, 256 to Violette et al, both assigned to the assignee of the present application, provide examples of such designs.
Blade retention may also be achieved by using a pin assembly at the base of the blade for attaching the same to a receiver portion attached to the hub of the propeller assembly. A plurality of patents has issued with respect to such designs. Examples of such designs are shown in U.S. Pat. No. 5,163,817 to Violette et al, U.S. Pat. No. 5,02,824 to Violette et al and U.S. Pat. No. 4,877,376 to Sikorsky et al.
Sikorsky et al is particularly interesting and describes a method of attachment of a rotor blade of fiber reinforced plastic to a metal rotor hub. The shank of the propeller blade attaches to the hub through a connection. Prestress can be applied through the connection to the propeller to assist in resistance of lateral impact force. In this design, the blade 4 connects to a tubular body B of the hub by means of bolts or screws. In addition, the blade includes a tubular shank 18b that extends into the tubular body and which further connects to the tubular body via an attachment or tensioning means 5b. The tensioning means 5b comprises a tensioning bolt 21 rotatably arranged in a transverse bore in shank 18b. The tensioning bolt includes pivot pins 22 and 23 extending from each end thereof and which are eccentric to the centerline of the bolt. Each pivot pin 22 and 23 resides in a support disk 25 and 26, respectively, which mount in circumferential slots 27 and 28, respectively, in the wall of tubular body 8b. When turning the tensioning bolt 21 via head 24 of pin 23, support disks 25 and 26 travel in slots 27 and 28. By this process, tension is applied to shank 19 in a direction of arrow 29 to press the blade against an upper flange of body 8b. This tensioning produces a prestress in the blade 4 that opposes stresses developed in the blade due to lateral impact forces during operations. While a cam or a center pin design is used to apply the downward force to the shank 18b, no locking mechanism is provided for maintaining the prestress on the shank. This shortcoming could potentially allow loss of the prestress and/or require that the bolts 6 maintain the prestress, which may also give rise to a prestress loss by their loosening.
An alternative embodiment is shown in FIG. 4 and in this embodiment, a retaining pin 32 resides in a transverse opening 31 in shank 18c. The pin includes end portions received in holes 34a and 34b in a tension sleeve 33 that surrounds shank 18c. The tension sleeve includes two spirally ascending semi-annular cam faces 35 and 36 that inversely conform to cam faces on an additional coaxially arranged rotary sleeve 37. The tension sleeve and rotary sleeve are arranged one above the other for axial displacement in the tubular body 8c. Tubular body 8c includes circumferential slots 38a and 38b to permit access to positioning holes 39a and 39b in the rotor sleeve 37. A tool, not shown, may be used to engage the positioning holes 39a and 39b such that the rotary sleeve 37 is rotatable to tension the shank 18c. Similar to the embodiment discussed above, there appears to be no means for locking the prestress position of rotary sleeve 37 and tension sleeve 33 to ensure maintenance of the preload. Again, the design uses bolts 36 to lock the preload position into place, which as discussed above, presents questionable preload security.
There exists a need, therefore, for an improved pin type retention and preload assembly for retaining a propeller blade to a hub and for preloading the same, whereby the preload position of the blade relative the hub is locked into place.